The present invention relates to a system and method for producing an electromagnetic (EM) field within the earth. More particularly, the invention relates to the generation of an electrical field at substantial depth and over substantial area using a cased borehole. The embodiments described herein relate generally to electromagnetic (EM) soundings within the earth based upon electric currents and the resulting electric and magnetic fields produced by those currents. As used herein, “earth” generally refers to any region in which a borehole may be located including, for example, the lithosphere.
EM geophysical soundings probe electrical resistivity in the earth as a function of depth. Typical targets of interest include ore bodies, hydrocarbons, water, proppants, hydraulic fracture (fracking) fluids, salts and other substances injected into the ground to improve the effectiveness of geophysical soundings as well as environmental pollutants. Since the resistivities of such targets and the surrounding medium may be quite dissimilar, it is possible to discriminate between them by means of measurement of their subsurface resistivity when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest may be determined.
The source of the EM field used in a geophysical sounding may originate in the natural environment, or be manmade. If manmade, the source is comprised of a transmitter and electrodes that make contact with the earth. The transmitter produces an oscillating voltage of the desired time-dependent waveform, which induces an electrical current to flow in the earth. Current is passed into the earth via a source electrode and returned to the transmitter via a counter electrode. However, the direction of current flow is in general oscillatory, and it is equally true to say the current flows into the ground from the counter electrodes and out via the source.
A current induced in the earth via a transmitter produces a primary magnetic field and also an electric field due to the electrical resistance of the ground. When oscillatory, these fields produce secondary EM fields. For example a time-varying magnetic field induces an electric field. The electrical properties of the earth and rate of change of the field determine the relative magnitudes of the secondary and primary fields. The combination of primary and secondary fields results in combined electromagnetic interaction with the earth even for a source intended to produce a solely electric or magnetic field.
While the majority of EM geophysical soundings are performed with an EM source on the surface of the earth, a borehole can provide physical access to the subsurface. Connecting a geophysical transmitter to the earth via a borehole provides a way to produce EM fields within the earth at desired depths without the attenuation and uncertainties that that may result if the source fields originate from a source at the surface of the earth.
Borehole sources to date fall into three categories: a) well logging, for which the source and receiver are located in the same borehole, b) cross well electromagnetic, and c) borehole-to-surface electromagnetic (BSEM). Borehole sources used for well logging are designed to produce an EM field in the immediate vicinity of the borehole, typically in the rock on the order of 1 m outside the borehole. Cross well EM sources produce an EM field that is measured in an adjacent borehole, up to approximately 1 km away. As implemented to date, cross well sources generate a magnetic field that is measured by a magnetic sensor in the adjacent well. In the BSEM method the source is within a borehole and an array of EM sensors is arranged at the ground surface. To date BSEM surveys have employed an electric field source and electric field sensors at the earth's surface.
FIG. 1 illustrates a known configuration wherein a borehole electric field source 2 comprised of an electrode 10, termed the source electrode, is positioned at depth within a borehole B of a well W, and an electrode 20 at the ground surface S is disposed near to the well and acts as a counter electrode. A transmitter 30 produces a voltage that induces an electric current to flow between the source 10 and counter 20 electrodes. Part of this current flows within the earth, where it generates EM fields that are characteristic of the electrical properties of the local earth medium.
The conventional configuration of a source electrode at depth in a borehole and a counter electrode at the top of the borehole is convenient to implement but has the disadvantage that the electric current largely flows in a vertical direction. Typical current paths, and the associated parallel electric fields, are indicated by lines in FIG. 1 for purposes of illustration. The paths are only shown on one side of the borehole but it should be understood to pass with approximately azimuthal symmetry all around the borehole. The precise path of the current depends on the electrical conductivity of the earth, which in general varies with both depth and azimuth about the borehole. However, regardless of the specific paths taken by the current, locating the counter electrode adjacent to the borehole minimizes the lateral projection of the current away from the borehole. As a result, the EM field that is generated in the earth decreases rapidly with increasing large lateral distance from the well.
One innovation to extend the lateral range of the BSEM configuration is to locate a number of counter electrodes 20′ at a distance from the well W′ in order of the depth of the source electrode 10′, and at least not less than 10% of the borehole depth. This advance is described in the recently filed patent application PCT/US12/39010: System and Method to Measure or Generate an Electrical Field Downhole, by Hibbs and Glezer, and illustrated in FIG. 2. The current is forced to flow laterally through the ground (i.e., orthogonal to a vertical borehole) by a distance at least equal to the radial distance between the source and counter electrodes. This configuration can be termed a radially grounded source (RGS). Still, a potential disadvantage of the BSEM method concerns the need for the borehole to be opened and a wireline lowered to the source electrode at the desired depth. An additional potential concern about the BSEM configuration is the electrical voltage that is present on the casing.
In any case, given the known prior art, it is desired to improve on the known prior art arrangements, particularly avoiding the need to wire a source electrode arranged deep within a well.